clk: rockchip: Fix SPIF special clock definition

Neither spdif_src nor spdif_pll exists, judging by the vendor kernel in
both cases spdif_pre was meant. This brings the naming in line and
hierachy in line with that of sclk_i2s0.

Also allow sclk_spdif and spdif_frac to change their parents rate as
that the upstream dividers are purely there to feed sclk_spdif

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>